They are nice. Most bags for say -5 to -10 performance seem to be in the 1.2-15kg weights. Zpacks and Kuiu seem to be closer to 500-600grams. Only thing holding me off a quilt type is you can't sit up in the tent in your bag...but this is a small thing.

I bought a quilt on a lighterpack mission and thought it would be a sacrifice from the bag, but I actually really like the system. Very flexable system. The only caveat I've found is that you want to use a strapping system when it gets down below 0. They are no hassle though and I think still easier than trying to get a mummy bag all cinched down

Gave the bag a run in Yosemite the other night, was meant to be down around zero but clouds rolled in so probably 3 degrees at the coldest?

I had thermals and fleece pants on as a couple months ago I got caught out and we just about suffered hyperthermia, was the coldest I have ever been and woken by pain and the sensation of my legs shutting down is an experience I wish never to repeat.

Was super warm, too warm with the pants on, just sleeping on the ground (no mat).
Bag design is awesome with lots of stretch and it's not a suffercating design so allows lots of wiggle room which makes for a better nights sleep. I'm about 6 foot so went for the long model which was primo for extra room and didn't feel like I lost any heat retention because of it.

Personally I'm not a huge fan of the small foot box and I like a full length zip so if I get too hot I can turn it in to a blanket. I generally like to be cold when I'm sleeping, just not near death cold